1. Field of the Invention
This invention relates to apparatus for CVD processing of a semiconductor wafer. More particularly, this invention relates to a shield which provides frontside edge exclusion of deposited materials on a semiconductor wafer during CVD processing as well as preventing backside depositions.
2. Description of the Related Art
In the deposition of materials on a semiconductor wafer during the formation of an integrated circuit structure, it is desirable to exclude deposited materials from the backside of the wafer, as well as from the edges of the front surface and the end edges of the wafer. This is of particular importance where the deposited material does not adhere to the native oxides usually present on such surfaces, which may require prior processing of such surfaces to promote adhesion.
For example, when tungsten is deposited over a silicon oxide insulation layer on a semiconductor wafer by CVD processing, the oxide surface must be pre-treated before the tungsten being deposited will properly adhere to the surface, e.g., by depositing a titanium tungsten (TiW) or titanium nitride (TiN) layer on the oxide surface. When tungsten deposits on the edges of the front surface of the wafer, or on the backside of the wafer, (which surfaces have not been pretreated with TiW or TiN) the deposited tungsten does not adhere properly, and flakes off as particles. As is well known, the generation of particles has a deleterious effect on subsequent wafer processing.
As shown in FIG. 1, in prior art deposition apparatus for CVD processing of a wafer, deposition gas or gases, containing a material such as tungsten to be deposited onto the front or top surface of a semiconductor wafer 10, enters the CVD chamber through a gas inlet or "showerhead" 20 which as shown is positioned above the wafer 10. A pumping ring 24 is located on a supporting lip or shoulder 26 of the chamber and has an inner diameter (ID) selected, with respect to the outer diameter (OD) of a circular susceptor 30 on which wafer 10 rests, to control the flow therebetween of a nonreactive purging gas from beneath the susceptor 30. The purpose of the purging gas flow is to inhibit passage of the deposition gas toward the edge and/or backside of the wafer, i.e., to help prevent unwanted deposition on such surfaces.
Unfortunately, however, it has been found that even when using such prior art apparatus, materials such as tungsten may still deposit on the frontside edges, end edges, and backside of the semiconductor wafer during deposition by CVD processing.
Studley et al, U.S. Pat. No. 4,932,358, in addressing this problem, discloses a seal ring which presses down against a wafer on a CVD chuck continuously around the outer periphery of the wafer and with sufficient force to hold the backside of the wafer against the chuck. The seal ring has one surface which contacts the front of the wafer and a second surface that extends close to the chuck, which is larger than the wafer, so that the edge of the wafer is also excluded from CVD coating. However, a complicated mounting mechanism is required to move the seal ring in and out of clamping engagement with the wafer and to maintain alignment between the seal ring and the wafer. Further, the seal ring can only be as wide as the diameter of the chuck.
It would, therefore, be desirable to provide simplified seal ring means which would engage a semiconductor wafer to protect the edges and backside of the wafer from undesirable depositions on such surfaces of the wafer.